Method of bonding titanium to stainless steel

ABSTRACT

A method of bonding a stainless steel part to a titanium part by heating a component assembly comprised of the titanium part, the stainless steel part, and a laminated titanium-nickel filler material placed between the two parts and heated at a temperature that is less than the melting point of either the stainless steel part or the titanium part. The component assembly is held in intimate contact at temperature in a non-reactive atmosphere for a sufficient time to develop a hermetic and strong bond between the stainless steel part and the titanium part. The bonded component assembly is optionally treated with acid to remove any residual free nickel and nickel salts, to assure a biocompatible component assembly, if implanted in living tissue.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/336,596, filed Jan. 20, 2006, now allowed, which is a divisional ofU.S. patent application Ser. No. 10/821,023, filed Apr. 7, 2004, nowabandoned.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a side view of the component assembly with fillermaterial as a foil between the stainless steel part and the titaniumpart.

FIG. 2 schematically depicts the bonding steps of the present invention.

FIG. 3 presents an isometric view of a titanium-nickel laminated fillermaterial having three foil layers.

FIG. 4 presents an isometric view of a titanium-nickel laminated fillermaterial having five foil layers.

FIG. 5 illustrates the compact filler material comprised of discreteparticles of titanium and nickel.

FIG. 6 presents a cross-sectional view of a discrete particle of nickeland titanium layers.

FIG. 7 presents an exploded isometric view of a ceramic tube, titaniumpart, and stainless part.

FIG. 8 illustrates a bonded device with a crimp-attached wire.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 presents component assembly 2 having a titanium part 4, astainless steel part 6, and a filler material 8. Component assembly 2 isheated to a specific process temperature that is below the melting pointof titanium part 4 or of the melting point of stainless steel part 6,for a specific period of time, at a pressure that is created by force10, that is exerted to place filler material 8 in intimate contact withthe titanium part 4 and stainless steel part 6.

Filler material 8 is preferably a laminate metal foil having a thicknessof approximately ten-thousandths (0.010) of an inch and more preferablyless than 0.010 inches. Filler material 8 is selected from the group ofmaterials that are compatible with the stainless steel chosen forstainless steel part 6 in that they wet the surface during the bondingprocess and enter into a diffusion process with the stainless steel part6, thereby creating a strong bond joint during processing. Fillermaterial 8 is further selected from the group of materials that arecompatible with the titanium part 4. Filler material 8 forms a bondbetween titanium part 4 and stainless steel part 6 at the bondingtemperature and pressure utilized during processing. The group of fillermaterials that are compatible with both the stainless steel part 6 andthe titanium part 4 includes substantially pure titanium and nickellaminate compositions, preferably comprised of filler materials of about22% to 98% nickel and the balance titanium. In a preferred embodiment,FIG. 3, filler material 8 is preferably comprised of alternating foillayers 12 and 14. Preferably, for example, as shown in FIG. 3, alaminate stack of commercially pure nickel layer 12 on the top outersurface 42 and a similar nickel layer 12 on the bottom outer surface 44.Sandwiched between the nickel layers 12 is a titanium layer 14. Thenickel layer 12 having at least 99.0% nickel and less than 1.0% of otherelements with a thickness of greater than approximately 0.0003 inchesand the titanium layer 14 comprised of commercially pure titanium foilhaving at least 99.0% titanium and less than 1.0% of other elements witha thickness of greater than approximately 0.0003 inches.

The inventors prefer the term “laminated” versus other descriptive, butequally applicable, terms such as “layered”, “clad”, or “composite”material. The laminated filler material is not an “alloy” of nickel andtitanium. An alloy, which is defined as a homogeneous mixture of two ormore metals, where the atoms of one replace or occupy interstitialpositions between the atoms of the other, of nickel and titanium, forexample, does not demonstrate the depressed melting point that isavailable at a eutectic composition when nickel and titanium are inintimate contact. The laminate material supplies substantially purenickel to initiate bonding with other metals, such as titanium orstainless steel, for example, at relatively low eutectic temperatures.For example, the lowest liquid us temperature (also referred to hereinas the melting point) in the nickel-titanium phase diagram occurs at 28%by weight nickel and is 942° C. Therefore, the optimum braze temperaturewill be greater than this temperature.

In a further preferred embodiment, FIG. 4, the metal foil layers 15,15′, and 15″, which are comprised of nickel, are placed in laminatedfiller material 8 as the top outer surface 42 and as the bottom outersurface 44, thereby making the nickel available to react directly withthe stainless steel part 6 and the titanium part 4. Alternating layersof inner mating foil layer 17 and 17′, which are comprised of titanium,are placed between the metal foil layers 15, 15′, and 15″.

Those skilled in the art know that the total composition of a laminatestack of alternating nickel and titanium foil is controlled by thethickness of the foil layers, where the volume fraction of nickel andtitanium is converted to weight percent by accounting for the density ofthe nickel and titanium. For example, to achieve a total laminate stackcomposition of a filler material 8 having a composition of 50 weightpercent Ni and 50 weight percent Ti, where the density of nickel is 8.90g/cc and of titanium is 4.51 g/cc, the thickness of the filler material8 will be 33.6% Ni foil and 66.4% Ti foil.

Titanium part 4 may be comprised of a titanium alloy and is comprised ofTi-6Al-4V, i.e. an alloy of titanium with 6 weight percent aluminum and4 weight percent vanadium, in a preferred embodiment. Stainless steelpart 6 may be comprised of one of the corrosion resistant stainlesssteels, such as, 304 stainless steel, or a 200, 300, or 400 seriesstainless steel, and in a preferred embodiment stainless steel part 6 iscomprised of 316L stainless steel. This configuration of componentsoffers the advantage of being biocompatible and of being capable offorming hermetic seals.

In an alternate embodiment, rather than using filler material 8 as afoil, filler material 8 may be a thin coating that is applied to thebonding surface of either the titanium part 4 or stainless steel part 6by any of a variety of chemical processes, such as electroless platingand electroplating, or by any of a variety of thermal processes, such assputtering, evaporating, or ion beam enhanced deposition.

In another embodiment, filler material 8 is applied as a thin coating ofmetallic beads, metallic powder, or discrete particles. The coating maybe applied in any of several methods known to those skilled in the art,such as painting, spraying, or dipping. The applied coating consists ofdiscrete particles of nickel and of titanium that aid in bonding thestainless steel part 6 and the titanium part 4 during the braze process.

In a further alternate embodiment, a compact filler material 8′, FIG. 5,is comprised of a bonded compact of primary alloy particulate 16 andsecondary alloy particulate 16′, where primary alloy particulate 16 ispreferably comprised of a nickel alloy and primary alloy particulate 16′is preferably comprised of a titanium alloy. The compact filler material8′ is formed by any of several techniques that are known to one skilledin the art, including cold pressing, warm pressing, slurry preparation,etc. The intimate mixture of primary alloy particulate 16 and secondaryalloy particulate 16′ bond together as well as react with the stainlesssteel part 6 and the titanium part 4 during the braze operation to yielda bonded component assembly 2.

Yet another alternate embodiment of forming a bonded component assembly2 utilizes the compact filler material 8′, as presented in FIG. 5, thatis comprised of layered discrete particle 19, preferably spheres,comprised of layered or laminated composition, as shown in FIG. 6. In apreferred embodiment, layered discrete particle 19 is comprised ofalternating layers of primary particle laminate layer 18 and secondaryparticle laminate layer 40, where primary particle laminate layer 18 ispreferably comprised of nickel and secondary particle laminate layer 40is comprised of titanium. The overall bonding methods and processes areanalogous to those employed for the several embodiments.

The process steps that are employed to create component assembly 2 witha strong bond between titanium part 4 and stainless steel part 6 areschematically represented in FIG. 2. First, the surfaces to be bondedare prepared in step 20 by machining to assure that they will intimatelyconform to each other during bonding. The surfaces are smoothed andcleaned.

In step 22, component assembly 2 is prepared with filler material 8between titanium part 4 and stainless steel part 6. In step 24, force 10is applied to compress filler material 8 between titanium part 4 andstainless steel part 6. Force 10 is sufficient to create intimatecontact between the parts. Force 10 is applied to assure that a bond isformed between titanium part 4 and stainless steel part 6, thus creatinga hermetic seal between the two parts. It is preferred that theresulting pressure be greater than about five psi.

In step 26, the assembly to be heat processed is placed in a furnace ina non-reactive atmosphere, which is preferably vacuum, but which, in analternative embodiment, can be any of several atmospheres that are knownto those skilled in the art, such as argon, nitrogen or hydrogen. Anon-reactive atmosphere is applied before the furnace is heated to theprocessing temperature in step 28. A preliminary holding temperature maybe utilized to allow the thermal mass of the parts to achieveequilibrium before proceeding with heating. In a preferred embodiment,the vacuum is less than 10⁻⁵ torr, to assure that the filler material 8and titanium part 4 do not oxidize. Component assembly 2 is held at theselected temperature, which is between approximately 940° and 1260° C.,for approximately 5 to 60 minutes, while force 10 continues to beexerted on filler material 8. The exact time, temperature and pressureare variable with each other so as to achieve a strong bond betweentitanium part 4 and stainless steel part 6. For example, in a preferredembodiment, a 316L stainless steel part is bonded to a Ti-6Al-4V part invacuum at 10⁻⁶ torr at approximately 1000° C. for 10 minutes with apressure of about 50 psi on a nickel-titanium foil of approximately0.002 inches total thickness.

The furnace is cooled and component assembly 2 is cooled to roomtemperature in step 30. In optional step 32, component assembly 2 iscleaned by being placed in a bath, after thermal processing is complete,to assure removal of all nickel and nickel salts. This bath ispreferably an acid bath that etches the exposed surfaces of componentassembly 2. In a preferred embodiment, the bath is nitric acid. Removalof nickel and nickel salts in the etch bath insures that componentassembly 2 is biocompatible. Nickel and nickel salts are detrimental toliving animal tissue. It is preferred that all of the nickel that isintroduced as filler material 8 is combined with the titanium and ischemically tied up by thermal processing to be unavailable as freenickel or as a nickel salt. Component assembly 2 is biocompatible afterbonding and processing.

In a preferred embodiment, component assembly 2 is either an electricalsensor or an electrical stimulator that is implanted in a human body,although it could equally well be implanted in any animal. It mustsurvive long periods in the hostile environment of a living body, whichis basically a warm saline solution. In a preferred embodiment,component assembly 2 is either a sensor or stimulator comprised of ahollow ceramic tube 36, containing various electronic components, thatis bonded to a titanium electrode end. The component assembly must bewatertight, resisting salt-water intrusion as well as growth of livingtissue into the titanium-to-stainless steel braze joint. FIG. 7 presentsan exploded isometric view of a ceramic tube 36 that is bonded to atitanium part 4 and a stainless steel part 6. The stainless steel part 6is designed to accept an electrically conductive wire, for transmissionof electrical signals.

Further, component assembly 2 does not corrode while implanted in thebody. The materials are chosen such that post-bonding they are notsusceptible to corrosion either individually or in the as-bonded state.Component assembly 2 resists electrolytic corrosion as well as crevicecorrosion, because of the materials selected for construction ofcomponent assembly 2.

A bonded device 52 is presented in FIG. 8 wherein a ceramic tube isbonded to titanium part 4 which is bonded to stainless steel part 6 witha filler material at braze joint 46. Stainless steel part 6 contains areceiver 54 into which a wire 50 is inserted and attached, preferably bycrimping, such that crimp indentation 48 retains wire 50. The bondeddevice 52 provides good electrical conductivity via stainless steel part6 connecting to wire 50. Stainless steel part 6 is brazed to titaniumpart 4 that is bonded by known methods to ceramic tube 36.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

1. A method of making a stainless steel and a titanium componentassembly for implantation in living tissue comprising the steps of:placing a filler material between a stainless steel part and a titaniumpart; compressing said filler material, forming intimate contact betweensaid stainless steel part and said titanium part by said compressing;said filler material having a melting point that is lower than themelting point of said titanium part or of said stainless steel part;bonding said component assembly, comprising said stainless steel part,said titanium part and said filler material, at a known temperature, fora predetermined time, said known temperature less than said meltingpoint of said titanium part or said stainless steel part, but greaterthan said melting point of said filler material; selecting saidstainless steel part from the group consisting of 200, 300, and 400series stainless steel; selecting said titanium part from the groupconsisting of titanium and titanium alloys, wherein the improvementcomprises: selecting said filler material to be a laminated fillermaterial comprised of at least one foil layer of titanium and at leastone foil layer of nickel; selecting said bonding temperature between940° and 1260° C.; and implanting said component assembly in a humanbody.
 2. The method of claim 1 comprising: selecting a 316L stainlesssteel part; and selecting a titanium part that is comprised ofTi-6Al-4V.
 3. The method of claim 1 further comprising: selecting alaminated filler material that is less than about 0.010 inches thickthat is comprised of a 22% to 98% nickel portion and a remainingtitanium portion, said laminated filler material comprising at least twonickel layers surrounding at least one titanium layer, and cooling thebonded component assembly.
 4. The method of claim 1 wherein compressingcreates compression between about 5 and 50 psi.
 5. The method of claim 1wherein compressing creates compression between about 5 and 7 psi. 6.The method of claim 1 further comprising forming said filler materialfrom metallic particulate.
 7. The method of claim 1 further comprisingbonding the assembly in a non-reactive atmosphere of a vacuum less than10⁻⁵ Torr.
 8. The method of claim 1 further comprising bonding theassembly in a non-reactive atmosphere of argon gas.
 9. The method ofclaim 1 wherein said predetermined time is between 5 and 60 minutes. 10.The method of claim 1 comprising cleaning said component assembly afterbonding to remove elemental nickel and nickel salts.
 11. The method ofclaim 10 additionally comprising cleaning said component assembly afterbonding by placing it in an acid bath.